System and method for plasmonic control of short pulses in optical fibers

ABSTRACT

The present disclosure relates to an optical waveguide system. The system has a first waveguide having a core-guide and a cladding material portion surrounding and encasing the core-guide to form a substantially D-shaped cross sectional profile with an exposed flat section running along a length thereof. The core-guide enables a core-guide mode for an optical pulse signal having a first characteristic, travelling through the core-guide. A material layer of non-linear material is used which forms a second waveguide. The material layer is disposed on the exposed flat section of the cladding material portion. The material layer forms a plasmonic device to achieve a desired coupling with the core-guide to couple optical energy travelling through the core-guide into the material layer to modify the optical energy travelling through the core-guide such that the optical energy travelling through the core-guide has a second characteristic different from the first characteristic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional and claims priority of U.S. patentapplication Ser. No. 16/037,837 filed on Jul. 17, 2018. The entiredisclosure of the above application is incorporated herein by reference.

STATEMENT OF GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

FIELD

The present disclosure relates to optical fibers, and more particularlyto systems and methods for controlling optical fiber properties throughthe use of plasmonic structures formed or secured on an optical fiber.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Plasmonic light-waves are electromagnetic waves propagating on metalsurfaces coupled with surface electron oscillations. The coupling toelectron oscillations enables extreme modifications to the propagatinglight, but this comes at a price of enhanced attenuation. However,careful design of complex metal-optic structures is a key-enabler formany ground breaking technologies. Merging plasmonics and optical fibertechnologies has been previously explored, but primarily for sensingapplications.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In one aspect the present disclosure relates to an optical waveguidesystem. The system may include a first waveguide having a core-guide anda cladding material portion surrounding and encasing the core-guide. Thecladding material portion forms a substantially D-shaped cross sectionalprofile with an exposed flat section running along a length thereof. Thecore-guide enables a core-guide mode for an optical pulse signal havinga first characteristic, travelling through the core-guide. A materiallayer of non-linear material is included for forming a second waveguide.The material layer is disposed on the exposed flat section of thecladding material portion. The material layer forms a plasmonic deviceto achieve a desired coupling with the core-guide to couple opticalenergy travelling through the core-guide into the material layer, suchthat the optical energy travelling through the core-guide has a secondcharacteristic different from the first characteristic.

In another aspect the present disclosure relates to a surface emittingoptical fiber. The surface emitting optical fiber may include an opticalfiber forming a first waveguide, and having a core-guide and a claddingmaterial portion surrounding and encasing the core-guide. The core-guideenables a core-guide mode for an optical signal having a first pulseprofile travelling through the core-guide. A second waveguide is securedto an outer surface of the first waveguide. The second waveguide forms aplasmonic device which implements a plasmonic mode waveguide. Theconstruction of the second waveguide is such as to achieve a desiredlevel of coupling between the core-guide mode and the plasmonic modewaveguide such that optical energy coupled into the second waveguideresults in a second pulse profile being different from the first pulseprofile, which is emitted out from the second waveguide.

In still another aspect the present disclosure relates to a method fortransmitting optical energy. The method comprises injecting opticalenergy forming a pulse having a first temporal pulse profile into acore-guide of an optical fiber. The core-guide forms a first waveguide,and the optical fiber has a cladding material portion with a D-shapedprofile. The method further includes using a non-linear material layersecured to the D-shaped profile of the cladding material portion to forma second waveguide. The second waveguide couples at least a portion ofthe optical energy out from the first waveguide such that the opticalenergy travelling through the first waveguide is modified to have asecond temporal pulse profile different from the first temporal pulseprofile.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a high level perspective view of one embodiment of a system inaccordance with the present disclosure illustrating a portion of anoptical fiber having a metallic plasmonic device patterned thereon;

FIG. 2 is a simplified side cross sectional view of the optical fiber ofFIG. 1;

FIG. 3A is a simplified diagram showing an end cross section of theoptical fiber of FIG. 1 with low coupling occurring between the coremode and with the lossy channel (i.e., the plasmonic mode/device) at ahigh intensity;

FIG. 3B is a simplified diagram of the optical fiber of FIG. 3A but witha high coupling occurring with the loss channel formed by the plasmonicdevice at a low intensity;

FIG. 3C is a graph illustrating that as the intensity increases, thecoupling shown in FIGS. 3A and 3B is designed to be reduced by thechange in the nonlinear index in between the two waveguides, therebyresulting in increased transmission for the low intensity parts of theoptical pulse;

FIG. 3D shows an another embodiment of the system of FIG. 1 in whichresonant cavities are formed on the flat surface of the D-shaped opticalfiber at spaced apart locations along the length of the optical fiber;

FIG. 4 shows a side view of another embodiment of the system of thepresent disclosure in which a plasmonic device formed by a metallicportion with a plurality of grooves extending normal to the length ofthe optical fiber may be used to form a side emitting optical fiber;

FIG. 5 shows another embodiment of the present disclosure in which anoptical fiber is arranged in a coiled configuration with a plurality ofaligned, side emitting segments metal segments disposed thereon, whichcan be used to create a two dimensional array of coherently added laserswith a total emitter area much larger than the optical fiber core;

FIG. 6 shows another embodiment of the present disclosure in which theoptical fiber of FIG. 4 (shown in simplified side view in FIG. 6) isused in connection with a broad area diode and a mirror positionednormal to the axis of propagation of an input wave through thecore-guide mode, to enable optical energy to be coupled into thecommon-mode guide of the optical fiber;

FIG. 7 is a simplified end cross sectional view of the optical fiber andmirror of FIG. 6; and

FIG. 8 is a simplified side view of another embodiment of the presentdisclosure showing a plurality of pump sources (e.g., broad area diodes)used to provide pump beams which are coupled into the core-guide of anoptical fiber at a plurality of locations along the length of theoptical fiber.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

Referring to FIGS. 1 and 2, a system 10 in accordance with oneembodiment of the present disclosure is shown. In this example thesystem 10 may comprise a length of D-shaped optical fiber 12 having acladding 12 a with a flat surface portion 12 b, and a core-guide 12 cencased within the cladding 12 a. The core-guide 12 c is a glass fiberthat enables a core-guide mode when an optical input signal 16 is inputinto the core-guide 12 c. A relatively thin metal layer 14 is secured orformed on the flat surface portion 12 b of the cladding 12 a. The thinmetal layer 14 forms a plasmonic device (hereinafter “plasmonic device14”), which supports a plasmonic mode when the optical signal 16 istravelling through the core-guide 12 c.

In the D-shaped optical fiber 12 such shown in FIGS. 1 and 2, thecore-to-fiber surface distance is reduced in a specific region, asindicated by dimension D1 in the side cross sectional view of FIG. 2,enabling the enhanced coupling to the plasmonic device 14. The thinmetal layer which forms the plasmonic device 14 may be, for example,constructed from, for example and without limitation, gold (Au), silver(Ag) or copper (Cu), and be on the order of between about tens ofnanometers to hundreds of nanometers in thickness, or possibly eventhicker. In this example the plasmonic device 14 forms an integrated,lattice-like structure having a plurality of spaced apart strips 14 awhich form grooves 14 b therebetween. The grooves 14 b in this exampleare formed normal to a longitudinal axis of the core-guide, indicated byline “A” in FIG. 2. The strips 14 a in this example may be spaced apartby a distance of up to hundreds of micrometers, or possibly evengreater, and each strip has a width that will be selected based on thespecific application, in one example up to hundreds of micrometers inwidth, or possibly even greater. In some applications, the thin metallayer forming the plasmonic device 14 may have no grooves at all. Theplasmonic device 14 may be created/applied through any suitable,well-known process, for example, by masked evaporation, sputtering,lithography or even controlled plating. However, it will be appreciatedthat the precise construction of the plasmonic device 14 may be tailoredto a specific application to best achieve specific desired performanceor results. The plasmonic device 14 supports a plasmonic local mode (oralternatively a lossy waveguide made with absorbing medium). Bymodifying the structural properties of the metal layer forming theplasmonic device 14, in connection with the cross sectional D-shape ofthe optical fiber 12 core structure, and the core spacing (i.e., D1 inFIG. 2), the coupling of optical energy into the plasmonic device 14from the core-guide 12 c can be precisely tuned as the input opticalsignal 16 travels through the core-guide 12 c. In this manner the energylevel of the optical signal 16 travelling through the core-guide 12 ccan be controlled (and will usually depend on the optical frequency),such as, for example, to create a controlled spectral transmission. Inone example the controlled spectral transmission may be used tocontrollably attenuate the optical signal travelling through thecore-guide 12 c.

One valuable application of the system 10 may be as a frequency notchfilter. The propagation in an optical fiber could be simplisticallyviewed in a ray optics description as a ray zig-zag bouncing inside thefiber due to total internal refractions. At each frequency the raypropagation angle is different (representative of the waveguide modalwavenumber, k-vector). The curve that details the k-vector of thepropagating mode as a function of the frequency is the dispersion curvecharacterizing the waveguide. The metal layer forming the plasmonicdevice 14 (i.e., being a plasmonic waveguide) has a different dispersioncurve than that of the fiber core (i.e., the core-guide mode 12 c). Whenthe two waveguides (i.e., plasmonic device 14 and core-guide 12 c) areput close together, the coupling between their modes is created, and atcertain frequencies the angle of propagation of the two matches better,which results in enhanced coupling (i.e., more optical energytransferred to the plasmonic device 14, as indicated by waveform 16 a inFIG. 2). At the frequency of enhanced coupling to the plasmonic mode(i.e., the plasmonic device 14), the attenuation of the propagating modewill therefore be enhanced, resulting in a notch filter. Since the notchlocation depends on structural properties, this sets the basis for apulse spectrum reshaping scheme by cascading two or more filters. Bytuning the difference between the wave-vector resonance of the plasmonicwaveguide formed by the plasmonic device 14 and that of the fiberwaveguide formed by the core-guide mode 12 c, and setting thenonlinearity in the coupling of the two such that there is less coupling(thus less loss) when the intensity increases, fast pedestal suppressionfunction (at the speed of the nonlinear effect) can be achieved. In thisregard it will be appreciated that the “pedestal” referred to representsthe lower intensity parts of the pulse away from its central peak. Thefast pedestal suppression is shown in FIG. 3C. FIG. 3C also shows thatat high intensity, indicated by point 20, for the coupling of FIG. 3A,fast pedestal suppression is achieved. As shown in FIG. 3B, to achievepedestal suppression function, the coupling at low intensity is set tobe high, resulting in a low transmission from the core-guide mode 12 cto the plasmonic device 14. As the intensity increases, the coupling,“α”, is designed to be reduced by the change in the nonlinear indexbetween the two waveguides—resulting in increased transmission, as shownin FIG. 3C by the curve 22 and the difference between the low intensitypoint 24 and the high intensity point 20. The system 10 is thusimplementing a pedestal suppression function, and the details of thefunction are being set by the nonlinear optical coupling scheme betweenthe two waveguides formed by the core-guide 12 c and the plasmonicdevice 14. Such a “passive optical valve” component can improve thecontrast between the “ON” state and the noise level. The nonlinearitywould result from the deposited nonlinear material layer 15 making upthe plasmonic device or from the metal nonlinearity itself. This conceptof designing a nonlinear optical transmission function to an opticalfiber segment could be further explored to obtain other complextransmission shapes, for example by depositing plasmonic cavities ontothe flat surface of the D-shaped fiber instead of a uniform thicknessmetal layer, which adds dispersion curve resonances. An additionalexample is a saturable absorber function resulting in a flat top shapedpulse.

The above teachings for designing the nonlinear transmission could befurther extended to affecting the accumulated phase. Similar to howspatially modifying the index (e.g., lens), and thus spatially theaccumulated phase, could reshape the light spatially, temporal reshapingof the phase could reshape the pulse in time. The coupling between thetwo waveguides formed by the core-guide 12 c and the plasmonic device 14modifies the intensity profile and, due to the optical nonlinearity,results in a modified refractive index. The net modal index (related tothe phase accumulation of the propagating mode) could be estimated asthe overlap integral of the modified refractive index and the fielddistribution shape. Therefore to obtain a negative b-integral, moreenergy should be guided at lower refractive index parts of the waveguide(clad) at higher intensities. This is a non-typical material responsethat could be designed into the system 10 using the structural approachdeveloped above. In this scheme, the coupling and the nonlinearity inthe plasmonic device 14 (i.e., effectively the plasmonic ‘cladding’) maybe designed such that at high intensity, more power is wave-guided atlower net index, which results in a negative Kerr effect and allows fora b-integral compensator for a laser system's front-end. The mainexisting solutions for front-end pulse shaping with sufficiently fastresponse presently suffer from being based on bulk components andlimited by properties of a given set of available materials. Forexample, a b-integral compensator could be implemented using KTP crystalnear the phase matching angle (through a cascaded khi-2 nonlinearity).The system 10, modified as described above, would have the advantage ofbeing an in-fiber integrated device, and have the wavelengthconfigurability based on the design.

Still another function that could be tailored using the system 10, whichforms plasmonic fibers, is unique fiber dispersion. As shown in FIG. 3D,this optional configuration could be achieved by forming resonantcavities 18 deposited onto the flat surface 12 b of the D-shaped fiber12, and could be even further enhanced by strong coupling of plasmonicstructures and cavities with excitons in dye molecules and quantum dots.In one example, the dye molecules are lossy at a given narrow bandwavelength, and could replace the metal material as the lossy media, oralternatively could be added to it, to thus reshape the response.

An efficient coupling scheme between the plasmonic mode and free spacefar-field would result in a side emitting optical fiber 100, as shown inFIG. 4. The optical fiber 100 includes a cladding 100 a having acore-guide mode 100 c and an outer surface portion 100 b. A carefullydesigned sub-wavelength metal array 102, having grooves 102 a, is knownto enable a maximum constructive interference in the far-field, that is,in the direction normal to the optical fiber 100 (i.e., indicated byarrows 104), which has been demonstrated for laser diodes facets.Optionally, the angle may be tuned as needed to meet a specificapplication; in other words the angle does not have to be normal to theoptical fiber 100. The side emitting plasmonic section formed by themetal array 102 could be repeated along the length of the optical fiber100 to create an array of emitters for emitting optical energy.Furthermore, each emitter could exhibit a different phase in acontrollable way, resulting in a phased array controlled beam. FIG. 5shows an embodiment of an optical fiber 200 coiled with a plurality ofaligned, side emitting segments metal segments 202 for emitting opticalenergy, such as shown in FIG. 4, which form plasmonic devices. Coilingof the optical fiber 200 creates a two dimensional array of coherentlyadded lasers, with total emitter area much larger than the area of thefiber core-guide 12 c.

Another potential modification may be the addition of a mirror 106, asshown in FIGS. 6 and 7, near the side emitter of the metal array 102.The lens-like shape of the D-shaped fiber cross-section cladding 100 aof the optical fiber 100 may be used to couple the normal-to-fiberradiation 103, from a pump source 108, for example a broad area diodelaser, which is controlled by an electronic controller 109, into a fiberpropagating mode (i.e., into a core-guide mode). Broad-area laser diodeshave been recently proposed to set attractive pumping sources for fiberlasers due to their relatively high power (˜10 W). The fiber coupledbroad area diode configuration shown in FIGS. 6 and 7 is expected to beof high interest as a pumping scheme since it allows coupling of thepump light from one or more optical pump sources into the longitudinallypropagating light 110 in the core-guide 100 c of the optical fiber 100.This configuration furthermore enables combining the power of severalpump sources 108 providing optical pump energy 108 a at spaced apartlocations along the length of an optical fiber 100, as shown in theembodiment of FIG. 8. Surface emitting fiber lasers have been suggestedin previous work but have typically involved using a complex structureof hollow core fiber filled with a gain medium and radial dielectricmulti-layer side walls. The system shown in FIG. 6 is constructed withsignificantly fewer component elements and allows for spatial control ofthe emitted beam.

The various embodiments described herein enable control over opticalfiber properties by patterning a plasmonic structure (or plasmonicstructures) directly onto optical fibers. Using the strong light-matterinteraction of plasmonics enables the design of unique pulse shapingfunctions and/or filtering to be achieved, as well as allowing for theconstruction of side emitting and pumping of fiber lasers.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure. Exampleembodiments are provided so that this disclosure will be thorough, andwill fully convey the scope to those who are skilled in the art.Numerous specific details are set forth such as examples of specificcomponents, devices, and methods, to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to thoseskilled in the art that specific details need not be employed, thatexample embodiments may be embodied in many different forms and thatneither should be construed to limit the scope of the disclosure. Insome example embodiments, well-known processes, well-known devicestructures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

1. An optical waveguide system including: a first waveguide having acore-guide and a cladding material portion surrounding and encasing thecore-guide to form a substantially D-shaped cross sectional profile withan exposed flat section running along a length thereof, the core-guideenabling a core-guide mode for an optical pulse signal having a firstcharacteristic, travelling through the core-guide; and a material layerof non-linear material forming a second waveguide, the material layerbeing disposed on the exposed flat section of the cladding materialportion, the material layer forming a plasmonic device to achieve adesired coupling with the core-guide to couple optical energy travellingthrough the core-guide into the material layer, which modifies theoptical energy travelling through the core-guide to cause the opticalenergy travelling through the core-guide to have a second characteristicdifferent from the first characteristic.
 2. The system of claim 1,wherein the first predetermined characteristic comprises a first opticalpulse shape, and the second characteristic comprises a second pulseshape, and wherein the first and second pulse shapes differ from oneanother.
 3. The system of claim 1, wherein the first characteristiccomprises a first energy level, and the second characteristic comprisesa second energy level, and wherein the first and second energy levelsdiffer from one another.
 4. The system of claim 1, wherein the materiallayer forms a lossy waveguide.
 5. The system of claim 1, wherein aplurality of the second waveguides are disposed along a length of theoptical fiber.
 6. The system of claim 1, further comprising anadditional plasmonic device forming a lattice like structure having aplurality of spaced apart strips held in an arrangement with a fixedspacing, the additional plasmonic device being disposed on a surface ofthe material layer.
 7. The system of claim 6, wherein the spaced apartstrips form grooves therebetween.
 8. The system of claim 7, wherein thegrooves are further formed normal to a longitudinal axis of thecore-guide.
 9. The system of claim 1, wherein the core-guide is arrangedin a coil, and wherein the second waveguide includes a plurality ofindependent plasmonic devices aligned adjacent to one another on theouter surface, which collectively form a two dimensional emitter. 10.The system of claim 1, wherein the second waveguide is constructed fromat least one of: copper; gold; and silver.
 11. A surface emittingoptical fiber including: an optical fiber having a core-guide and acladding material portion surrounding and encasing the core-guide, thecore-guide forming a first wave-guide and enabling a core-guide mode foran optical signal having a first pulse profile travelling through thecore-guide; a second waveguide secured to an outer surface of the firstwaveguide, the second waveguide forming a plasmonic device whichimplements a plasmonic mode waveguide; the construction of the secondwaveguide being such as to achieve a desired level of coupling betweenthe core-guide mode and the plasmonic mode waveguide such that opticalenergy coupled into the second waveguide has a second pulse profilebeing different from the first pulse profile, which is emitted out fromthe second waveguide.
 12. The system of claim 11, wherein the claddingmaterial portion comprises a D-shaped profile having a flat portion, andwherein the second waveguide forms a planar element secured to the flatportion.
 13. The system of claim 11, further comprising a plurality ofthe second waveguides disposed along a length of the cladding materialportion and spaced apart from one another.
 14. The system of claim 13,wherein the cladding material portion is arranged in a coil, and theones of the plurality of second waveguides are arranged to be alignedalong the coil.
 15. The system of claim 12, further comprising anadditional plasmonic device forming a lattice like structure having aplurality of spaced apart strips held in an arrangement with a fixedspacing, the additional plasmonic device being disposed on the flatportion of the D-shaped profile.
 16. A method for transmitting opticalenergy comprising: injecting optical energy forming a pulse having afirst temporal pulse profile into a core-guide of an optical fiber, thecore-guide forming a first waveguide, and the optical fiber having acladding material portion with a D-shaped profile; and using anon-linear material layer secured to the D-shaped profile of thecladding material portion to form a second waveguide, the secondwaveguide coupling at least a portion of the optical energy out from thefirst waveguide such that the optical energy travelling through thefirst waveguide is modified to have a second temporal pulse profiledifferent from the first temporal pulse profile.